There are currently two major routes available for the preparation of encapsulated nanoparticles inside protein cages:
There are the self-assembly of the cages in presence of synthesized and appropriately functionalized nanoparticles and the in-situ synthesis of nanoparticles inside protein cages (Aniagyei S. E. et al., J. Mater. Chem., 2008, 18, 3763-3774).
The self-assembly route inherently requires the protein cage to be able to undergo reversible disassembly and reformation in presence of the nanoparticles, i.e., once the protein cage is broken up it should be possible to reconstitute it back. The method is clearly not applicable to protein cages that disassemble irreversibly.
The in-situ growth of nanoparticles inside protein cages, although by far the most popular method, has limits related to the growth of nanoparticles in confinement. In most cases accurate control over the morphology of the nanoparticles is impaired. Surface effects become dominant and the bulk properties of the assemblies display broader distributions. For example, growth induced from multiple nucleation sites results in polycrystalline nanoparticles, which in case of semiconductor nanocrystals results in inferior optical properties compared to presynthesized quantum dots (QDs). Growing passivating inorganic shells like in CdSe/ZnS is virtually impossible due to the restricted access to the surface of the nanoparticles after the synthesis. One would also desire a material where the stoichiometric ratio between the nanoparticle and the protein cage is equal to 1. This is in particular important in particle tracking, bioimaging and biosensing, where the knowledge of stoichiometry would allow one to perform quantitative analysis.
There is a need to provide new methods for encapsulating the nanoparticles in protein cages to overcome the above mentioned limitations.